Bank segment - Components of investment securities (Details) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015 USD ($) issue	Dec. 31, 2014 USD ($) issue
Available-for-sale securities
Total available-for-sale securities, Amortized cost	$ 779,080	$ 549,627
Gross unrealized gains	9,319	6,745
Gross unrealized losses	(2,562)	(5,978)
Available-for-sale securities	$ 785,837	$ 550,394
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues | issue	12	12
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	$ 81,939	$ 88,999
Gross unrealized losses, less than 12 months	$ (324)	$ (525)
Available for Sale, Securities, More Than 12 Months, Number of Issues | issue	28	34
Fair value, 12 months or longer	$ 151,092	$ 201,791
Gross unrealized losses, 12 months or longer	(2,238)	(5,453)
U.S. Treasury and federal agency obligations
Available-for-sale securities
Total available-for-sale securities, Amortized cost	209,025	119,507
Gross unrealized gains	2,435	1,092
Gross unrealized losses	(342)	(1,039)
Available-for-sale securities	$ 211,118	$ 119,560
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues | issue	4	6
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	$ 24,676	$ 41,970
Gross unrealized losses, less than 12 months	$ (46)	$ (361)
Available for Sale, Securities, More Than 12 Months, Number of Issues | issue	3	5
Fair value, 12 months or longer	$ 18,218	$ 29,168
Gross unrealized losses, 12 months or longer	(296)	(678)
Mortgage-related securities- FNMA, FHLMC and GNMA
Available-for-sale securities
Total available-for-sale securities, Amortized cost	570,055	430,120
Gross unrealized gains	6,884	5,653
Gross unrealized losses	(2,220)	(4,939)
Available-for-sale securities	$ 574,719	$ 430,834
Available-for-Sale, Securities, Less Than 12 Months, Number of Issues | issue	8	6
Available-for-sale Securities, Continuous Unrealized Loss Position
Fair value, less than 12 months	$ 57,263	$ 47,029
Gross unrealized losses, less than 12 months	$ (278)	$ (164)
Available for Sale, Securities, More Than 12 Months, Number of Issues | issue	25	29
Fair value, 12 months or longer	$ 132,874	$ 172,623
Gross unrealized losses, 12 months or longer	$ (1,942)	$ (4,775)